Developing device

ABSTRACT

A developing device having a toner carrier for feeding a toner through pressure contact with a latent image carrier on which an electrostatic latent image pattern has been formed. The toner carrier comprises a foam member having a foam portion and a solid surface layer portion. The foam portion and the solid surface layer portion comprise an identical material and are continuous with each other substantially without the presence of any interface therebetween. The foam member has a density gradient in the direction of the thickness. The developing device of the present invention enables a soft pressure contact development to be stably conducted, has advantageously low production and operation cost, and can form an image having a high resolution without a significant variation in the density.

This is a continuation of Ser. No. 08/046,074, filed Apr. 12, 1993,which is a continuation of application Ser. No. 07/854,291, filed Mar.19, 1992 now U.S. Pat. No. 5,214,239).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for forming an imagethrough the use of an electrophotographic process. More particularly,the present invention is concerned with a developing device wherein atoner carrier is brought into pressure contact with a latent imagecarrier to develop an image.

2. Description of the Related Art

As disclosed in U.S. Pat. No. 3,731,146, a conventional developingdevice known in the art, in which a toner carrier is brought intopressure contact with a latent image carrier to develop an image,comprises a foam member as a backing material (a base material) and asoft electroconductive layer separately provided on the surface of thefoam member to form a toner carrier. In this device, a toner istransported to the toner carrier and brought into pressure contact witha latent image carrier to develop an image.

In the above-described conventional toner carrier, however, in order toform an even electroconductive layer on the surface of a foam member asa base material, it is at least necessary to form an electroconductivelayer so that the layer thickness is greater than the size of a cell ofthe foam member. However, it is not always easy to retain theflexibility of the foam member up to the surface of the toner carrierand exhibit the flexibility in that place. For this reason, afluctuation in the environment or the like gives rise to a variation inthe development pressure and developing nip width (contact width), sothat there occur problems of a fluctuation in the density and a loweringin the resolution. Further, in the above-described toner carrier havinga structure comprising two separate layers, that is, a base material anda surface layer, it is at least necessary to provide, in the manufacturethereof, a series of steps, i.e., at least a step of forming a foammember, a step of machining the foam member and a step of forming anelectroconductive layer. Further, since the foam member is flexible,particularly the step of machining the foam member becomes complicatedand inevitably requires a lot of time, which gives rise to an increasein the production cost of the toner carrier and in its turn an increasein the cost of the developing device.

SUMMARY OF THE INVENTION

The present invention solves the above-described problems of the priorart. An object of the present invention is to provide a developingdevice which enables a soft pressure contact development to be stablyconducted through the use of a soft foam member. Another object of thepresent invention is to provide a developing device which provides ahigh resolution and is less liable to give rise to a fluctuation in thedensity of the image. A further object of the present invention is toprovide a developing device provided with a toner carrier which is easyto manufacture and at low cost.

The developing device of the present invention has a toner carrier forfeeding a toner through pressure contact with a latent image carrier onwhich an electrostatic latent image pattern has been formed, whereinsaid toner carrier comprises a foam member having a foam portion and asolid surface layer portion, said foam portion and said solid surfacelayer portion comprising an identical material and being continuous witheach other substantially without the presence of any interfacetherebetween, said foam member having a density gradient in thedirection of the thickness.

According to the above-described constitution of the present invention,the toner carrier comprises a foam member comprised of a foam portionand a solid surface layer portion made of the same material as that ofthe foam portion. The foam portion and the solid surface layer comprisean identical material and are continuous with each other substantiallywithout any interface therebetween, and the foam member has a densitygradient in the direction of the thickness. Thus, a toner carrierparticularly having a high-density solid surface layer portion can beprovided. This makes it possible to prepare a toner carrier which caneliminate the necessity of complicated post-machining and is inexpensiveand, at the same time, enables a soft pressure contact development to beconducted through the utilization of the softness of the foam portionand the flexibility of the solid surface layer portion, so that itbecomes possible to provide a developing device which is less liable togive rise to damage to a toner and has a high durability.

The above-described constitution can provide a developing device havinga simple structure wherein use is made of a one-component non-magnetictoner.

The provision of an electroconductive layer on the solid surface layerportion enables a developing electrode to be made proximate to thelatent image carrier, -so that it becomes possible to form an imagehaving a high resolution without a significant edge effect. Since anelectroconductive layer is formed on the toner carrier, the toner can beheld and transported by means of an electrostatic image force at thetime of the transportation of the toner, so that it is possible toprovide a developing device having a simple structure wherein use ismade of particularly a one-component nonmagnetic toner.

Further, when a magnetic field generating layer is provided on the solidsurface layer portion, the toner is restrained on the toner carrier bymagnetic force to stabilize the transportation of the toner, so thatfogging (deposition of the toner on a non-image portion) can beinhibited by magnetic force.

Further, when the toner carrier is in a roller form and the solidsurface layer portion is provided at least on the outer periphery of thefoam member, it becomes possible to provide such an Outer shape facethat the polishing of an outer shape is unnecessary and functionallayers, such as an electroconductive layer and a magnetic fieldgenerating layer, can be easily formed on the solid surface layerportion.

Further, when the solid surface layer portion is provided in a portionto be fixed to a shaft, the fixing strength between the shaft and thefoam member can be improved.

Further, when the thickness of the solid surface layer portion is 2 mmor less, it becomes possible to obtain a flexibility sufficient forpressure contact development.

Further, when the foam member comprises a soft foam, an ample pressurecontact width can be obtained under a low development pressure, whichenables fluctuations in the pressure contact state at the pressurecontact portion to be reduced, so that the development density can bestabilized. In particular, the use of foam materials, such as apolyurethane foam, a polystyrene foam, a polyethylene foam, an elastomerfoam and a rubber foam, makes it possible to provide a developing devicewhich has an ample softness, is inexpensive and has excellent massproductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional view of an image forming apparatuswherein use is made of an embodiment of the developing device accordingto the present invention;

FIG. 2 is a schematic cross-sectional view of an image forming apparatuswherein use is made of another embodiment of the developing deviceaccording to the present invention;

FIG. 3 is a schematic cross-sectional view of an image forming apparatuswherein use is made of a further embodiment of the developing deviceaccording to the present invention;

FIGS. 4(a) and 4(b) are graphical diagrams showing density distributionsof a foam member of the developing device according to the presentinvention;

FIGS. 5(a), 5(b), and 5(c) are schematic cross-sectional views of anembodiment of the developing device according to the present inventionwherein an electroconductive layer is provided on the surface of a tonercarrier;

FIGS. 6(a), 6(b), 6(c), and 6(d) are schematic cross-sectional views ofanother embodiment of the developing device according to the presentinvention wherein a magnetic field generating layer is provided on thesurface of a toner carrier; and

FIGS. 7(a) and 7(b) are graphs indicating mechanical properties of amagnetic field generating layer provided on a toner carrier in thedeveloping device according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic cross-sectional view of an image forming apparatuswherein use is made of an embodiment of the developing device accordingto the present invention. A latent image carrier 1 comprises anelectroconductive support 2 and, provided thereon, a photosensitivelayer 3 comprising an organic or inorganic photoconductive material. Thephotosensitive layer 3 is electrified by an electrifier 4, such as acorona charger or an electrifying roller. Thereafter, light from a lightsource 5, such as laser or LED, is passed through an image formingoptical system 6 and selectively applied to the photoconductive layer 3corresponding to image information, thus causing a desired electrostaticlatent image pattern to be formed by a potential contrast thus caused.In a developing device 7, a toner 8 as an image forming material istransported to develop an image. A toner carrier 9 for transporting thetoner 8 comprises a shaft 10 and, provided on the outer periphery of theshaft 10, a foam member 13 having a foam portion 11 and a solid surfacelayer portion 12 which comprise an identical material and is continuouswith each other. A blade 14 comprising a nonmagnetic or magnetic metalis pressed against the toner carrier 9 to electrify the toner 8 to aspecific polarity and, at the same time, to regulate the toner layer toa suitable thin layer. The toner 8 is held directly on the toner carrier9, and the toner carrier 9 is rotated to transport the toner 8 in a thinlayer form. The toner carrier 9 is in contact with the latent imagecarrier 1 under a specific pressure. In the pressure contact portion orin the vicinity of the pressure contact portion, a developing electricfield is formed by a potential contrast of the latent image carrier 1and a developing bias voltage applied between the latent image carrier 1and the toner carrier 9 or between the latent image carrier 1 and theblade 14, and the electrified toner 8 is subjected to a developmentdepending upon the developing electric field. Thus, the electrostaticlatent image pattern of the latent image carrier 1 is visualized bymeans of the electrified toner 8. Further, the toner image istransferred onto a recording paper 16 through the use of a transferdevice 15, such as a corona transfer device or a transfer roller, andfixed on the recording paper 16 by heat or pressure to obtain a desiredimage on the recording paper 16.

FIG. 2 is a schematic cross-sectional view of an image forming apparatuswherein use is made of another embodiment of the developing deviceaccording to the present invention. In this figure, those parts whichare the same or have substantially the same functions as correspondingparts in FIG. 1 are designated by the same reference numerals. Adeveloping device 20 serves to transport a non-magnetic toner 8 toconduct a development. A foam member 23 having a foam portion 21 and asolid surface layer portion 22 which comprise an identical material andare continuous with each other are provided on the outer periphery ofthe shaft 10. An electroconductive layer 24 is provided on the solidsurface layer portion 22 of the foam member 23. A toner carrier 35comprises the shaft 10, the foam member 23 and the electroconductivelayer 24. The toner carrier 25 is in contact with the latent imagecarrier 1 under a specific pressure. In the pressure contact portion orin the vicinity of the pressure contact portion, a developing electricfield is formed by a potential contrast of the latent image carrier 1and a developing bias voltage applied between the latent image carrier 1and the electroconductive layer 24 of the toner carrier 25 or betweenthe latent image carrier 1 and the blade 14, and the electrified toner 8is subjected to a development depending upon the developing electricfield. Thus, the electrostatic latent image pattern of the latent imagecarrier 1 can be visualized by means of the electrified toner 8.

FIG. 3 is a schematic cross-sectional view of an image forming apparatuswherein use is made of a further embodiment of the developing deviceaccording to the present invention. In this drawing, the same referencenumerals designate parts which are the same or have substantially thesame functions as corresponding parts in FIGS. 1 and 2. A developingdevice 30 serves to transport a non-magnetic toner 8 to conduct adevelopment. A foam member 33 having a foam portion 31 and a solidsurface layer portion 32 which comprise an identical material and arecontinuous with each other are provided on the outer periphery of theshaft 10, and a magnetic field generating layer 34 is provided on thesolid surface layer portion 32 of the foam member 33. A toner carrier 35comprises the shaft 10, the foam member 33 and the magnetic fieldgenerating layer 34. The above-described constitution wherein the tonercarrier 35 has a magnetic field generating layer 34 enables the magnetictoner 8 to be directly held on the toner carrier 35 and transferred bymagnetic force onto the surface of the magnetic field generating layer34 through the use of a leakage magnetic field. The toner carrier 35 isin contact with the latent image carrier 1 under a specific pressure. Inthe pressure contact portion or in the vicinity of the pressure contactportion, a developing electric field is formed by a potential contrastof the latent image carrier 1 and a developing bias voltage appliedbetween the latent image carrier 1 and the toner carrier 35 or betweenthe latent image carrier 1 and the blade 14. The electrified magnetictoner 8 is subjected to a development depending upon the developingelectric field. Thus, the electrostatic latent image pattern of thelatent image carrier 1 is visualized by means of the electrifiedmagnetic toner 8.

In FIGS. 1 to 3, the photosensitive layer 3 of the latent image carrier1 may comprise an organic or inorganic photosensitive material. Theshaft 10 may comprise a metallic material, such as steel or aluminum, aresin material or the like. The blade 14 may comprise a metallic plateor a thin metallic sheet or a resin or a resin film having a shape of adoctor knife at its tip. In order to reduce the thickness of the tonerlayer on the toner carrier, the blade 14 is preferably formed in such amanner that it is pressed against the toner carrier. Further, the toner8 may comprise any known toner irrespective of whether or not itpossesses magnetism and may be either a resin toner or a wax toner. Thedeveloping agent is not limited to a one-component type.

The foam member for constituting the toner carrier will now bedescribed.

FIG. 4 comprises two graphical diagrams showing examples of densitydistribution of foam members of the developing device according to thepresent invention. FIG. 4(a) is a diagram showing the densitydistribution of a foam member according to an embodiment of the presentinvention. a foam portion is fixed to a shaft by means of vulcanizationbonding or the like. The foam portion has foamed cells continuous withone another and ample softness. In the vicinity of the outer periphery,the density distribution increases describing a parabola in the radialdirection, and a high-density solid surface layer portion comprising thesame material as that of the foam portion and having substantially nofoamed cell (vacancy) is formed on the outer periphery. Such a solidsurface layer portion can be formed by a method such as a reactioninjection molding method wherein a stock solution of a foaming materialis injected into a die for a reaction. The solid surface layer portionis formed to have a thickness of 2 mm or less, preferably of the orderof a number of hundreds μm. Thus, it is possible to form a toner carrierhaving ample flexibility and a smooth surface without detriment to thesoftness of the foam portion. The ratio of the thickness of the solidsurface layer portion to the thickness of the foam portion is preferablyin the range of from 0.02 to 0.5, still preferably in the range of from0.03 to 0.2. In this ratio, the pressure contact development can befurther stabilized. When the above ratio exceeds 0.5, the hardness ofthe surface layer portion increases, so that the flexibility becomesinsufficient. On the other hand, when the above ratio is less than 0.02,pin holes are unfavorably liable to occur on the surface of the surfacelayer portion.

FIG. 4(b) is a diagram showing the density distribution of a foam memberaccording to another embodiment of the present invention. A foam memberis formed on a shaft by means of vulcanization bonding or the like or bymonolithic molding together with the shaft. The foam portion has foamedcells continuous with one another and ample softness. In the vicinity ofthe outer periphery, the density increases substantially describing aparabola, and a high-density solid surface layer portion comprising thesame material as that of the foam portion and having substantially nofoamed cell (vacancy) is formed on the outer periphery. As with theembodiment shown in FIG. 4(a), the solid surface layer portion can beformed by a reaction injection molding method or the like. The solidsurface layer portion can be formed also on the shaft portion to improvethe fixing strength. Further, the shaft can be inserted into a die tocarry out monolithic molding, which makes it possible to form a tonercarrier in a small number of steps of working with high mechanicalprecision.

Although the foam portion may be in the form of either a closed cell oran interconnected cell, since the closed cell gives rise to a change inthe dimension with temperature, the interconnected cell is preferred.When the thicknesses of the foam portion and the solid surface layer areof the order of 2 to 10 mm and a number of hundreds μm, respectively,the toner carrier can be pressed against the latent image carrier by avery low pressure of 1 to 5 g/mm in terms of the linear pressureunattainable by conventional rubber, so that the pressure contactdevelopment can be conducted while maintaining a stable pressure contactstate without any damage to the toner. A soft foam material is favorableas the foam member, and examples thereof include a polyurethane foam, apolystyrene foam, a polyethylene foam, an elastomer foam and a rubberfoam. In particular, the polyurethane foam has excellent moldability anda high hydrophilic property, which renders the polyurethane foamsuitable for forming an electroconductive layer and a magnetic fieldgenerating layer on the solid surface layer portion. Further, the foammember can be produced by, besides the reaction injection moldingmethod, a gas inclusion method, a foaming agent decomposition method, asolvent volatilization method, a chemical reaction method, a sinteringmethod, an elution method, etc. The formation of the solid surface layerportion by any of the above methods affords high dimensional accuracy tobe attained without post-working, so that a toner carrier having a lightweight can be produced at low cost.

The formation of functional layers, such as an electroconductive layerand a magnetic layer, on the solid surface layer portion as shown inFIGS. 2 and 3 provides a functional roller having a high softness.Embodiments of the layer structure of the toner carrier will now bedescribed.

FIG. 5(a) is a schematic cross-sectional view of an embodiment of thedeveloping device according to the present invention wherein aconductive layer is formed on the surface of a toner carrier. A foamportion 51 and a solid surface layer portion 52 are formed in that orderon a base 50, such as a shaft, and an electroconductive layer 53 isformed on the solid surface layer portion. The provision of a conductivelayer 53 on the outer periphery portion of the toner carrier in this waymakes possible the attainment of a sufficient developing electrodeeffect by applying a developing bias voltage to the conductive layer 53through a thin toner layer at the time of the pressure contactdevelopment to make the developing electrode proximate to the latentimage carrier, so that it becomes possible to form an image having ahigh resolution and an excellent area gradation without a significantedge effect (which is a state wherein a strong electric field occurs atthe edge portion of the image and excess toner is subjected to adevelopment). With respect to the resistance of the electroconductivelayer 53, a stable developing current can be obtained when theresistance between the surface of the electroconductive layer 53 and thedeveloping bias voltage application portion is 100 MΩ or less. Thethickness of the electroconductive layer 53 is preferably 100 μm or lessfrom the viewpoint of ensuring sufficient flexibility. With respect tothe material for the electroconductive layer 53, the dispersion of anelectroconductive fine powder, such as carbon black or a metal, insubstantially the same resin as that in the foam portion 51 cancontribute to an improvement in the bonding strength between the solidsurface layer portion 52 and the electroconductive layer 53. An evenlayer can be formed as the conductive layer 53 through the use of spraycoating, transfer coating, in-mold coating, roller coating, electrolessplating or the like.

FIG. 5(b) is a schematic cross-sectional view of another embodiment ofthe developing device according to the present invention wherein aconductive layer is provided on the surface of a toner carrier. A solidsurface layer portion 52, a foam portion 51 and another solid surfacelayer portion 52 are provided in that order on a base 50, such as ashaft, and an electroconductive layer 53 is provided on the anothersolid surface layer portion 52. By the provision of an electroconductivelayer 53 on the outer periphery portion of the toner carrier in thisway, ample developing electrode effect can be attained. The bondingstrength between the shaft 50 and the foam member can be improved by theprovision of the solid surface layer portion 52 between the shaft 50 andthe foam portion 51, and the number of steps of working can be reducedthrough monolithic molding of the shaft 50 and the foam member. Thesolid surface layer portion 52 can be provided not only on the upper andlower sides of the foam portion 51 but also on the end of the foamportion 51 or the like. By the above described construction, theinclusion of toner in the foam portion 51 can be prevented.

FIG. 5(c) is a schematic cross-sectional view of a further embodiment ofthe developing device according to the present invention wherein aconductive layer is provided on the surface of a toner carrier. A solidsurface layer portion 52, a foam portion 51 and another solid surfacelayer portion 52 are formed in that order on a base 50, such as a shaft.An electroconductive layer 53 is provided on the another solid surfacelayer portion 52, and a protective layer 54 is provided on theelectroconductive layer 53. The provision of a protective layer 54 onthe electroconductive layer 53 in this way contributes to an improvementin the durability of the electroconductive layer 53 and, at the sametime, makes possible control of the polarity of electrification and theamount of electrification through the inclusion of a charge controlagent in the protective layer 54. The thickness of the protective layer54 is preferably 100 μm or less, more preferably of the order of severalμm from the viewpoint of ensuring ample flexibility. With respect to thematerial for the protective layer 54, a resin having excellent abrasionresistance, such as a fluororesin, is used in the case where animprovement in the durability is intended, while a dye, a pigment or thelike can be used in the case where the electrification of the toner iscontrolled.

FIG. 6(a) is a schematic cross-sectional view of an embodiment of thedeveloping device according to the present invention wherein a magneticfield generating layer is provided on the surface of a toner carrier. Inthis embodiment, a foam portion 61 and a solid surface layer portion 62are provided in that order on a base 60, such as a shaft, and a magneticfield generating layer 63 is provided on the solid surface layer portion62. When the magnetic field generating layer 63 is provided on the outerperiphery of the toner carrier in this manner, the magnetic toner can beheld on the toner carrier by means of magnetic force and stablytransported, so that the scattering of the toner can be prevented.Further, at the time of the development, the occurrence of fogging canbe reduced through the generation of the development inhibitory force bythe magnetic force against the developing force by the developingelectric field. Further, it is also possible to attain a developmentelectrode effect by making the magnetic field generating layer 63electroconductive. The thickness of the magnetic field generating layer63 is preferably 100 μm or less from the viewpoint of ensuring ampleflexibility. With respect to the material for the magnetic fieldgenerating layer 63, the dispersion of a ferromagnetic fine powder insubstantially the same resin as that in the foam portion 61 contributesto an improvement in the bonding strength between the solid surfacelayer portion 62 and the magnetic field generating layer 63. An evenlayer can be formed as the magnetic field generating layer 63 throughthe use of spray coating, transfer coating, in-mold coating, rollercoating, electroless plating or the like. The ferromagnetic material forthe magnetic field generating layer 63 may be any material known as amagnetic recording material and a magnetic material. Specific examplesthereof include a magnetic material containing at least one elementselected from Fe, Ni, Co, Mn and Cr, for example, γ-Fe₂ O₃, Ba-Fe,Ni-Co, Co-Cr and Mn-Al. The demagnetization of the magnetic fieldgenerating layer 63 caused by a magnetic toner and a magnetically softcomponent around the developing device can be prevented by applying acoercive force of 2000 Oe or more.

FIG. 6(b) is a schematic cross-sectional view of another embodiment ofthe developing device according to the present invention wherein amagnetic field generating layer is provided on the surface of a tonercarrier. In this embodiment, a solid surface layer portion 62, a foamportion 61 and another solid surface layer portion 62 are provided inthat order on a base 60, such as a shaft, and a magnetic fieldgenerating layer 63 is provided on the solid surface layer portion 62.When the magnetic field generating layer 63 is provided on the outerperiphery of the toner carrier in this manner, the magnetic toner can beheld on the toner carrier by means of magnetic force and stablytransported, which prevents scattering of the toner, so that theoccurrence of fogging can be reduced. The bonding strength between theshaft 60 and the foam member can be improved by the provision of thesolid surface layer portion 62 between the shaft 60 and the foam portion61, and the number of steps of working can be reduced through monolithicmolding of the shaft 60 and the foam member. The solid surface layerportion 62 can be formed not only on the upper and lower sides of thefoam portion 61 but also on the end of the foam portion 61 or the like.By the above-described construction, the inclusion of toner in the foamportion 61 can be prevented.

FIG. 6(c) is a schematic cross-sectional view of a further embodiment ofthe developing device according to the present invention wherein amagnetic field generating layer is provided on the surface of a tonercarrier. In this embodiment, a solid surface layer portion 62, a foamportion 61 and another solid surface layer portion 62 are provided inthat order on a base 60, such as a shaft. An electroconductive layer 64is provided on the solid surface layer portion 62, and a magnetic fieldgenerating layer 63 is provided on the electroconductive layer 64. Theprovision of an electroconductive layer 64 and a magnetic fieldgenerating layer 63 on the solid surface layer portion 62 in this mannerproduces ample developing electrode effect, so that it becomes possibleto form an image having a high resolution and an excellent areagradation. Further, since ample toner holding power can be obtained, itis possible to form an image having no significant fogging and a highcontrast. The thickness of the electroconductive layer 64 is preferably100 μm or less from the viewpoint of ensuring ample flexibility.

FIG. 6(d) is a schematic cross-sectional view of a further embodiment ofthe developing device according to the present invention wherein amagnetic field generating layer is provided on the surface of a tonercarrier. In this embodiment, a solid surface layer portion 62, a foamportion 61 and another solid surface layer portion 62 are provided inthat order on a base 60, such as a shaft. A magnetic field generatinglayer 63 is provided on the solid surface layer portion 62, and aprotective layer 65 is provided on the magnetic field generating layer63. The provision of a protective layer 65 on the magnetic fieldgenerating layer 63 in this way contributes to an improvement in thedurability of the magnetic generating layer 63 and, at the same time,makes possible control of the polarity of electrification and the amountof electrification through the inclusion of a charge control agent inthe protective layer 65. The thickness of the protective layer 54 ispreferably 100 μm or less, still preferably of the order of several μmfrom the viewpoint of ensuring sufficient flexibility. With respect tothe material for the protective layer 54, a resin having an excellentabrasion resistance, such as a fluororesin, is used in the case where animprovement in the durability is intended, while a dye, a pigment or thelike can be used in the case where the electrification of the toner iscontrolled.

Preferred examples of the method of forming a toner carrier having anelectroconductive layer on a solid surface layer portion according tothe above-described embodiments will be further described below.

At the outset, an electroconductive hollow cylindrical member ispreviously fabricated. A foam member having a foam portion and a solidsurface layer portion is formed within the cylindrical member to form atoner carrier. Examples of the hollow cylindrical member used in thiscase include a thin-walled cylindrical tube of a metal, for example, aNi tube fabricated by electroforming and an Al tube or a Cu tubefabricated by extrusion. Separately, an electroconductive resin tube ispreviously fabricated, and a toner carrier is formed by the same methodas that described above. In the resin tube in this case, use may be madeof any resin provided that it is flexible and moldable into a tube.Examples thereof include dispersions of electroconductive powders, suchas carbon black, in materials such as urethane, polyesters, nylon,fluororesins, polyethylene, polypropylene, vinyl chloride, silicone,acrylic rubber, chloroprene rubber, nitrile rubber, styrene-butadienerubber, polybutadiene and polyisoprene. Further, it is also possible touse a resin tube wherein an electroconductive material isvapor-deposited or coated on the surface of the tube. Further, the tubemay be composed of a material comprising an electroconductive resincomposition besides the dispersion of a conductive agent in a resin. Thetube can be prepared by a known method such as extrusion or rapping.Further, it is also possible to use a thermal shrinkage tube comprisingthese materials. Other examples of the method of forming anelectroconductive layer include a method wherein an Al, Cr or An metalis directly vapor-deposited on the solid surface layer portion.

When the electroconductive layer is formed on the solid surface layerportion, some materials for the foaming agent cause an additive to bleedfrom the lower layer, so that contamination or breaking of thephotoreceptor or filming of the toner is liable to occur. In this case,it is also possible to provide an intermediate layer, i.e., a bleedingpreventive layer, between the solid surface layer portion and theelectroconductive layer.

Favorable features which the toner carrier according to the presentinvention has will now be described.

The roughness of the toner carrier surface (the surface of theelectroconductive layer or protective layer if it is provided) ispreferably 20 μmRz or less, more preferably 10 μmRz or less.

In order to stably conduct a soft pressure contact development, thehardness of the toner carrier is preferably 10° to 70°, more preferably20° to 35° in terms of Asker C hardness. Asker C hardness is the broadlyused Japanese standard for hardness according to the regulation of arubber hardness meter manufactured by KOBUNSHI KEIKI K.K., Japan.

Since the toner carrier comprising a foam member is an elastomer, theoccurrence of a permanent strain in the toner carrier gives rise to afluctuation in the nip between the photoreceptor and the toner carrier,a fluctuation in the pushing pressure, etc. which becomes causative of aremarkable deterioration in the image. The permanent compression strain(as determined according to JIS-K-6301) of the toner carrier ispreferably 20% or less for the purpose of reducing the above-describeddrawbacks. Further, when the permanent compression strain is 5% or less,the deformation derived from the pressure contact of components witheach other can be substantially eliminated.

The magnetic field generation layer will now be described in moredetail.

In a preferred embodiment of the present invention, the magnetic fieldgenerating layer comprises a heat shrinkage sleeve comprising a magneticpowder dispersed in a binder. Examples of the binder usable in this caseinclude heat-shrinkable materials such as polyurethane, polyvinylchloride, polyethylene, polypropylene, straight-chain polyesters,polyamide resins, crystalline polyolefins, crosslinked polyolefins,non-crosslinked polyolefins, crystalline polyolefin copolymers,fluororesins, silicone rubber and EPDM. Other examples of the binderinclude, besides the polyurethane resin, vinyl copolymer resins such asvinyl chloride, fluororesins, polyethylene, polypropylene, polyesters,polyamide resins, polyolefins, epoxy resins, vinyl acetate, vinylalcohol, vinyl butyral, vinyl formal and vinyl ethyl ester,nitrocellulose, cellulose acetobutyrate, and blends thereof with otherresins. Further, all of other elastic or flexible materials, forexamples, rubber materials, such as EPDM, fluororubber, silicone rubber,chloroprene rubber, natural rubber, isoprene rubber, butadiene rubber,neoprene rubber and NBR, may be used as the binder.

The magnetic powder dispersed in the binder may be any magnetic powderknown as a magnetic recording material or a magnet material. Specificexamples thereof include magnetic materials containing at least oneelement selected from Fe, Ni, Co, Mn and Cr, for example, γ-Fe₂ O₃,BaO-6Fe₂ O₃, Co-γ-Fe₂ O₃, Ba-Fe, Ni-Co, Co-Cr and Mn-Al.

Besides the magnetic powder, electroconductive powders such as amagnetic powder, a metallic whisker and carbon black may be incorporatedin the binder. In this case, the carbon black preferably has acapability of forming a strong structure and serves to remarkably lowerthe electrical resistance with an increase in the amount of addition ofthe carbon black. Specific examples of the carbon black usable in thiscase include ketjenblack, acetylene black, lamp black and high structurefurnace black. Acetylene black or other carbon black having an excellentelectric conductivity is particularly favorable. Since the surface areaper unit amount of addition of the carbon black increases with areduction in the particle diameter, a carbon black having a smallparticle diameter is preferred for the purpose of lowering the electricconductivity.

The method of forming the above-described sleeve magnetic fieldgenerating layer will now be described.

The sleeve magnetic field generating layer can be formed by molding suchas extruding, injection molding, blow molding, vacuum molding,compression molding, transfer molding and centrifugal molding. Amongthem, the centrifugal molding wherein a molten resin is injected intothe inner surface of a cylindrical mold being rotated at a high speedand molded is particularly suitable when the surface of the sleeve isfinished to a specular state. Further, according to the extruding, sincea seamless sleeve having an even thickness can be continuously produced,the production cost can be reduced. When the surface roughness, Rz, ofthe extrusion mold is in the range of from 0.05 to 5 μm, since a desiredsurface roughness can be given to the surface of the sleeve, this isparticularly desired for the production of an inexpensive developingcarrier.

In the formation of the sleeve magnetic field generating layer, it isadvantageous to use a roller covered with a heat-shrinkable sleeve. Forexample, after a foam member is inserted into the sleeve before the heatshrinkage, the environment is evacuated and heated (to 120° C.) toshrink the sleeve to cover the outer periphery of the foam member. Thus,when a heat-shrinkable sleeve is shrunken after the environment isevacuated, it is possible to eliminate the problem of air stayingbetween the foam member and the sleeve and a protruded portion occurringon the surface of the sleeve.

Further, according to this method, since the heat-shrinkable sleeve iscaused to adhere to the foam member by the holding power of theheat-shrinkable sleeve, it becomes unnecessary to fix theheat-shrinkable sleeve to the foam member by means of an adhesive.Further, even when the foam member and the heat-shrinkable sleeve aresubjected to a bonding treatment, since the surface of the foam memberis covered with a relatively high-density solid surface layer portion,there is no possibility of the adhesive being impregnated into the cellof the foam portion to excessively raise the hardness of the foammember.

On the other hand, when the outer periphery of a coarse cell foam isdirectly covered with a heat-shrinkable sleeve, since theheat-shrinkable sleeve shrinks while tracing the inequality in the cellof the foam portion, the developing agent carrier obtained is one havinga very rough surface. On the other hand, in the above-described method,since the solid surface layer portion can be made very smooth bycovering the foam member having a solid surface portion on the outerperiphery of the foam with the heat-shrinkable sleeve, the developingagent carrier has a very smooth surface also after covering with theheat-shrinkable sleeve. When very small through holes scatter on thesolid surface layer portion, even though the step of evacuating theenvironment is omitted, there is no possibility of air remaining betweenthe heat-shrinkable sleeve and the solid surface layer portion.

In this method, after a foam member is inserted into the interior of theheat-shrinkable sleeve, heating is successively conducted from one endof the heat-shrinkable sleeve by heating means such as an electricfurnace or a drier to cover the sleeve with the outer periphery of thefoam member. In this method as well, since the covering of the foammember is successively conducted from the end of the sleeve, it ispossible to prepare a developing agent carrier having a smooth surfacewithout causing air to remain. Further, since the covering with thesleeve can be conducted without evacuation of the environment, the sizeof the covering device can be reduced. Further, since the covering timecan be shortened, the above method is suitable particularly for massproduction.

In the above described heat shrinking method, for example, after a foammember is inserted into the inside of the heat-shrinkable sleeve, theheat-shrinkable sleeve can be successively immersed into a hot waterbath of about 90° C. to conduct shrink covering.

The toner carrier can be produced by, besides the above-described methodwherein a heat-shrinkable sleeve is caused to adhere to the outerperiphery of the foam member, a method wherein a sleeve having amagnetic powder, an electroconductive powder or other powder dispersedtherein is inserted together with a shaft into a mold and monolithicmolding is conducted by reaction injection molding or the like. Thismonolithic molding makes possible production of a toner carrier in asmall number of manufacturing steps with high mechanical precision.

The method of magnetizing the toner carrier will now be described.

There is no particular limitation on the magnetization method, which canbe conducted by in-plane magnetization or vertical magnetization.

In the in-plane magnetization, the toner carrier is rotated with amagnetic head abutted against the magnetic field generating layer and,at the same time, the magnetic head is scanned towards the axialdirection of the toner carrier to conduct the magnetization. In thismethod, the magnetization is spirally conducted with a small pitch inthe circumferential direction of the toner carrier. In the toner carrieraccording to the present invention, since the magnetic field generatinglayer is formed on the foam member having elasticity, even when themagnetic head is strongly abutted against the magnetic field generatinglayer, the abutting pressure of the head is dispersed and relaxed, sothat damage to the magnetic field generating layer can be minimized andthe head gap portion is caused to adhere closely to the magnetic fieldgenerating layer. This prevents gap loss during the magnetization.

Examples of the vertical magnetization include magnetic copying whereinuse is made of a master magnet having a leakage magnetic flux density of2000 Gs on its surface. The master magnet in this case may be acylindrical Nd-Fe-B-base rare earth magnet having a thickness of 1 mm.In this magnet, the magnetization can be conducted with a 1-mm pitch sothat the leakage magnetic flux density of the magnet on its surface is2000 Gs.

In the above-described embodiment, the magnetic field generating layercan be formed by providing, on a foam member, a 200 μm-thick materialcomprising a dispersion of a γ-Fe₂ O₃ -base magnetic powder and carbonblack in a urethane binder. The magnetization of the magnetic fieldgenerating layer is conducted, for example, by magnetic copying whereinthe magnetic field generating layer and the master magnet are rotated atthe same peripheral velocity while they are abutted against each otherin such a manner that the contact width between the magnetic fieldgenerating layer and the master magnet is about 3 mm. Since the magneticfield generating layer is formed on the foam member, the toner carriercomes into contact even with a very hard master magnet while causing thetoner carrier to be elastically deformed without leaving a gap, so thatthe magnetization can be evenly conducted at any position of the tonercarrier.

When a sleeve having a magnetic powder dispersed thereon is used as amagnetic field generating layer, after the vertical magnetization ispreviously conducted on the sleeve, a foam member can be covered withthe magnetized sleeve or subjected to monolithic molding together withthe magnetized sleeve.

With respect to the thickness of the magnetic field generating layer,according to the results of studies we have conducted, it is apparentthat the magnetic restraint increases with an increase in the thicknessof the magnetic coating. In particular, a good magnetic restraint can beobtained when the thickness of the magnetic coating is 20 μm or more,preferably 40 μm or more.

With respect to the coercive force of the magnetic field generatinglayer, the larger the coercive force of the magnetic coating, thestronger is the magnetic restraint. When the coercive force is 200 Oe ormore, the magnetic restraint can be stably maintained withoutattenuation. Further, when the coercive force is 2,000 Oe or more, amagnetic restraint corresponding to a development bias of about 100 V isobtained, so that the occurrence of fogging derived from the tonerhaving an opposite polarity can be prevented.

Further, since the toner is held and transported by means of magneticforce, no lowering in the density derived from the insufficiency of theamount of feed of the toner is observed when printing is conducted at ahigh speed of 30 PPM or more. Further, even when the toner feed rolleris removed, an ample amount of toner can be fed by means of magneticforce alone. In this respect as well, the device can be produced at alow cost. Further, the toner feeding performance can be stabilizedindependently of the residual amount of the toner within a hopper.

In order to develop the above described magnetic restraint, it ispreferred that 0 to 60% by weight, preferably 30% by weight of amagnetic powder be incorporated in the toner.

The smaller the thickness of the sleeve having a magnetic powderdispersed thereon, the better is the flexibility. According to ourfindings, a flexibility usable in the development can be obtained whenthe thickness is 700 μm or less, and a particularly good flexibility canbe obtained when the thickness is 300 μm or less.

The surface roughness (Rz) and the coefficient of dynamic friction ofthe magnetic field generating layer is preferably in the range of from0.1 to 5 μm and in the range of from 0.2 to 0.7, respectively.

The abrasion resistance of the magnetic field generating layer will nowbe described.

FIG. 7(a) shows a change of the abrasion loss of a sleeve magnetic fieldgenerating layer comprising an acrylic resin as a binder with the elapseof time, and FIG. 7(b) shows a relationship between the abrasion lossand the surface roughness (Rz) with respect to the surface of a sleeve.Since the surface of the sleeve is always fretted through tonerparticles at a position facing the elastic blade and the electrostaticlatent image carrier, as shown in FIG. 7(a), the surface of the sleevewas abraded with the elapse of time. Consequently, as shown in FIG.7(b), the surface roughness (Rz) rapidly decreased with an increase inthe abrasion loss. In particular, since the abrasion loss of the sleeveimmediately after the initiation of use of the toner carrier is large,the variation of the amount of transportation of the toner at an initialstage became large.

Therefore, when a sleeve having a low abrasion resistance is provided onthe surface of the toner carrier, although the amount of transportationand the amount of electrification of the toner are favorable at aninitial stage for the development, the surface of the sleeve isunfavorably abraded during sliding of the toner on the surface of thesleeve, so that an image having an insufficient density is unfavorablyformed when the surface roughness of the sleeve and the coefficient offriction are not favorable for the transportation and electrification ofthe toner. Further, when a flaw or the like occurs in the sleeve, sincethe amount of transportation of the toner becomes heterogeneous, thereoccurs a severely fogged image or an image having a lowered fine linereproducibility.

We have made further studies based on the above described findings and,as a result, have found that when the material for the sleeve coveringthe elastic layer exhibits an abrasion loss of 700 mg or less asdetermined by an abrasion test (abrasion wheel H22, rolling abrasion in1,000 revolutions) specified in JIS-K7311, it is possible to prepare atoner carrier wherein the surface of the sleeve is not easily abradedwhen it is subjected to severe rubbing and friction by the developingagent, and the surface roughness remains constant over a long period oftime, so that an image having ample image density and a good resolutioncan be stably formed. Further, when the material for the sleeve exhibitsan abrasion loss of 200 mg or less as determined by the above describedabrasion test, the abrasion of the sleeve at an initial stage can beremarkably reduced. Therefore, it became possible to form an image withlittle fluctuation in the density from the initiation of use of thetoner carrier over a long period of time.

The formation of a magnetic field generating layer in the abovedescribed manner increases the Asker C hardness by 2° or more,preferably 5° or more, by which a layer having a high hardness can beformed on the surface thereon. It is possible to attain the effect ofpreventing stick slipping in the blade portion and the developmentportion through curing of the surface in the above described hardnessrange.

In the present invention, it is also possible to provide a bleedoutpreventive effect between the foam member and the magnetic fieldgenerating layer. The bleedout preventive layer, which serves to preventthe occurrence of bleedout of a leachable substance, comprises anoil-resistant layer of a resin or a rubber, and it is preferred that thesolubility parameter, SP, of the resin or rubber constituting theoil-resistant layer and the solubility parameter, SPs, of theplasticizer contained in the foam member satisfy the requirementrepresented by the following formula:

    2≦|SP-SPs|

Further, the magnetic field generating layer per se can be used as thebleedout preventive layer.

According to the present invention, when the above describedelectroconductive layer and the magnetic field generating layer areformed on the solid surface layer portion by a coating method, sincesubstantially no infiltration of the coating into the foam memberoccurs, it becomes possible to form an electroconductive layer and amagnetic field generating layer each having high adhesiveness. Also inthe case of the formation of the electroconductive layer and magneticfield generating layer in a tubular form, the adhesion of these tubescan be easily and strongly accomplished for the abovedescribed reason,and it is possible to form a toner carrier having an even surface.

EXAMPLES

The present invention will now be described more fully with reference tothe following examples, although it is not limited to these examplesonly.

Example 1

A shaft and a foam member of polyurethane foam were integrally moldedthrough the use of polyurethane as a main starting material by reactioninjection molding to form a toner carrier having an outer diameter of 20mm and provided with a solid surface layer portion on the outerperiphery thereof. The surface of the toner carrier after molding mayhave any roughness depending upon the surface state of the mold. Asufficiently smooth surface having a surface roughness of 5 μm or lesscould be obtained by polishing the mold, and it was unnecessary toconduct post-working such as polishing of the outer shape of the tonercarrier. The weight of the toner carrier except for the shaft could bereduced to 25 g when the toner carrier had a length of 230 mm and anouter diameter of 60 mm. When this toner carrier was pressed against alatent image carrier having an outer diameter of 60 mm by a pressure of2 g/mm, the developing nip width (contact width) was about 2 mm. Thus, adeveloping device capable of maintaining a soft and stable pressurecontact state could be prepared. This developing device was used for animage forming apparatus as shown in FIG. 1, and a line image of 300 DPI,a character image and a solid image were continuously formed on 10,000sheets of recording paper. As a result, the 300 DPI-image was stablyobtained without suffering from thickening of the line image, and asolid image having an excellent area gradation and a high density of 1.4or more in terms of the OD value could be stably formed. Further,neither unfavorable phenomena such as fixation and fusing on the latentimage carrier or toner carrier nor damage to the toner was observed. Inthe formation of an image, a developing bias voltage was applied to theblade, and deelectrification was conducted after the development.

Example 2

A shaft and a foam member of a silicone rubber foam were integrallymolded by reaction injection molding through the use of anelectrifying-agent-containing silicone rubber as a main startingmaterial to form a toner carrier having an outer diameter of 20 mm andprovided with a solid surface layer portion on the outer peripherythereof. The surface of the toner carrier after molding had amplesmoothness, and it was unnecessary to conduct post-working such aspolishing of the outer shape of the toner carrier. The resistancebetween the surface of the toner carrier and the shaft was of the orderof several MΩ, that is, a suitable electric conductivity, could beobtained, and development could be conducted with a developing biasvoltage applied to the shaft. When this .toner carrier was pressedagainst a latent image carrier having an outer diameter of 60 mm with apressure of 2 g/mm, the developing nip width (contact width) was about 2mm. Thus, a developing device capable of maintaining a soft and stablepressure contact state was obtained. This developing device was used foran image forming apparatus as shown in FIG. 1, and a line image of 600DPI, a character image and a solid image were continuously formed on10,000 sheets of recording paper. As a result, the 600 DPI line imagewas stably obtained without a significant edge effect and thickening ofthe line image, and a solid image having a good dot recordingreproducibility, an excellent area gradation and a high density of 1.4or more in terms of the OD value was stably formed. Further, neitherunfavorable phenomena such as fixation and fusing on the latent imagecarrier or toner carrier nor damage to the toner was observed. In thisexample, an electroconductive fine particle of carbon black or the likewas used as the electrifying agent, and a toner carrier having anarbitrary resistance could be formed by varying the carbon blackcontent.

Example 3

A shaft and a foam member of a polyurethane foam were integrally moldedby reaction injection molding through the use of polyurethane as a mainstarting material, and an electroconductive coating comprising carbonblack dispersed in polyurethane as a binder was spray-coated on a solidsurface layer portion as a surface layer to form a coating having athickness of about 50 μm, thereby forming an electroconductive tonercarrier having an outer diameter of 20 mm. The surface of the tonercarrier after molding had ample smoothness, and it was unnecessary toconduct post-working such as polishing of the outer shape of the tonercarrier. Sufficient flexibility and compression set could be obtainedeven when the toner carrier was pressed against a latent image carrierhaving an outer diameter of 30 mm. Further, the surface resistance ofthe toner carrier was 1 MΩ or less, that is, a suitable electricconductivity could be obtained, and development could be conducted witha developing bias voltage applied to the shaft. When this toner carrierwas pressed against a latent image carrier having an outer diameter of30 mm with a pressure of 3 g/mm, the developing nip width (contactwidth) was about 2 mm. Thus, a developing device capable of maintaininga soft and stable pressure contact state was obtained. This developingdevice was used for an image forming apparatus as shown in FIG. 2, and aline image of 600 DPI, a character image and a solid image werecontinuously formed on 1,000 sheets of recording paper. As a result, the600 DPI line image having a high resolution was stably obtained withouta significant edge effect and without thickening of the line image, anda solid image having a good dot recording reproducibility, an excellentarea gradation and a high density of 1.4 or more in terms of the ODvalue was stably formed. Further, neither unfavorable phenomena such asfixation and fusing on the latent image carrier or toner carrier nordamage to the toner was observed.

Example 4

A shaft and a foam member of a polyurethane foam were integrally moldedby reaction injection molding through the use of polyurethane as a mainstarting material, and an electroconductive coating comprising carbonblack dispersed in polyurethane as a binder was spray-coated on a solidsurface layer portion as a surface layer to form an about 50 μm-thickelectroconductive layer. Further, the electroconductive layer was coatedwith a fluororesin to form an about 1 μm-thick protective layer, therebyforming an electroconductive toner carrier having an outer diameter of20 mm. The surface of the toner carrier after molding had amplesmoothness, and it was unnecessary to conduct post-working such aspolishing of the outer shape of the toner carrier. Ample flexibility andcompression set was obtained even when the toner carrier was pressedagainst a latent image carrier-having an outer diameter of 30 mm. Whenthis toner carrier was pressed against a latent image carrier having anouter diameter of 30 mm with a pressure of 3 g/mm, the developing nipwidth (contact width) was about 2 mm. Thus, a developing device capableof maintaining a soft and stable pressure contact state was obtained.This developing device was used for an image forming apparatus as shownin FIG. 2, and a line image of 600 DPI, a character image and a solidimage were continuously formed on 2,000 sheets of recording paper. As aresult, the 600 DPI line image having a high resolution was stablyobtained without a significant edge effect and without thickening of theline image, and a solid image having a good dot recordingreproducibility, an excellent area gradation and a high density of 1.4or more in terms of the OD value was stably formed. Further, neitherunfavorable phenomena such as fixation and fusing on the latent imagecarrier or toner carrier nor damage to the toner was observed. Thedurability of the toner carrier was improved by the provision of aprotective layer.

Example 5

A shaft and a foam member of a polyurethane foam were integrally moldedby reaction injection molding through the use of polyurethane as a mainstarting material, and a magnetic electroconductive coating comprisingγ-Fe₂ O₃ and carbon black dispersed in polyurethane as a binder wasroller-coated on a solid surface layer portion as a surface layer toform a coating having a thickness of about 10 μm, thereby forming atoner carrier having an outer diameter of 20 mm and provided with amagnetic field generating layer. The surface of the toner carrier aftermolding had ample smoothness, and it was unnecessary to conductpost-working such as polishing of the outer shape of the toner carrier.The above-described toner carrier having a magnetic field generatinglayer was subjected to line magnetization with a magnetization reversingpitch of 50 μm in a very fine pitch, and a thin layer of a tonercomprising a magnetic toner (toner layer thickness: about 20 μm) couldbe held on the surface of the toner carrier by magnetic force. Further,sufficient flexibility and compression set could be obtained even whenthe toner carrier was pressed against a latent image carrier having anouter diameter of 30 mm. Further, the surface resistance of the tonercarrier was 1 MΩ or less, that is, a suitable electric conductivity wasobtained, and development was conducted with a developing bias voltageapplied to the shaft. When this toner carrier was pressed against alatent image carrier having an outer diameter of 60 mm by a pressure of2 g/mm, the developing nip width (contact width) was about 2 mm. Thus, adeveloping device capable of maintaining a soft and stable pressurecontact state was obtained. This developing device was used for an imageforming apparatus as shown in FIG. 3, and a line image of 600 DPI, acharacter image and a solid image were continuously formed on 10,000sheets of recording paper. As a result, a 600 DPI line image having ahigh resolution was stably obtained without a significant edge effectand without thickening of the line image. Thus a solid image havingneither tailing nor fogging in the end of the image and having a gooddot recording reproducibility, an excellent area gradation and a highdensity of 1.4 or more in terms of the OD value was stably formed.Further, no fogging occurred on the latent image carrier, not to mentionon the recording paper, so that the amount of a waste toner wasremarkably reduced. Thus, a reduction in the fogging by virtue ofmagnetic restraint was confirmed. Further, neither unfavorable phenomenasuch as fixation and fusing on the latent image carrier or toner carriernor damage to the toner was observed.

Example 6

A shaft and a foam member of a polyurethane foam were integrally moldedby reaction injection molding through the use of polyurethane as a mainstarting material, and a magnetic electroconductive coating comprisingγ-Fe₂ O₃ and carbon black dispersed in polyurethane as a binder wasroller-coated on a solid surface layer portion as a surface layer toform a 10 μm-magnetic field generating layer. Further, a fluororesin wascoated on the magnetic field generating layer to form a 1 μm-thickprotective layer, thereby forming a toner carrier having an outerdiameter of 20 mm and provided with a magnetic field generating layer.The surface of the toner carrier after coating of the protective layerhad ample smoothness, and it was unnecessary to conduct post-workingsuch as polishing of the outer shape of the toner carrier. Theabovedescribed toner carrier having a magnetic field generating layerwas subjected to magnetization with a very fine pitch, and a thin layerof a toner comprising a magnetic toner could be held on the surface ofthe toner carrier by magnetic force. Further, when this toner carrierwas pressed against a latent image carrier having an outer diameter of60 mm with a pressure of 2 g/mm, the developing nip width (contactwidth) was about 2 mm. Thus, a developing device capable of maintaininga soft and stable pressure contact state was obtained. This developingdevice was used for an image forming apparatus as shown in FIG. 3, and aline image of 600 DPI, a character image and a solid image werecontinuously formed on 20,000 sheets of recording paper. As a result,the 600 DPI line image having a high resolution was stably obtainedwithout a significant edge effect and without thickening of the lineimage, and a solid image having neither tailing nor fogging in the endof the image and having a good dot recording reproducibility, anexcellent area gradation and a high density of 1.4 or more in terms ofthe OD value was stably formed. Further, no fogging occurred on thelatent image carrier, not to mention on the recording paper, so that theamount of a waste toner was remarkably reduced. Thus, a reduction in thefogging by virtue of magnetic restraint was confirmed. Further, neitherunfavorable phenomena such as fixation and fusing on the latent imagecarrier or toner carrier nor damage to the toner was observed. Thus, thedurability of the toner carrier was improved by providing a protectivelayer.

Example 7

A shaft and a foam member of a silicone rubber foam were integrallymolded by reaction injection molding through the use of a siliconerubber containing an electrifying agent as a main starting material, andthe solid surface layer portion was subjected to a treatment with asilane coupling agent to form an adhesive layer. A magneticelectroconductive coating comprising γ-Fe₂ O₃ dispersed in a polyolefinas a binder was roller-coated on the solid-surface layer portion as asurface layer to form an about 10 μm-magnetic field generating layer.Further, a fluororesin was coated on the magnetic field generating layerto form a 1 μm-thick protective layer, thereby forming a toner carrierhaving an outer diameter of 20 mm and provided with a magnetic fieldgenerating layer. The surface of the toner carrier after coating of theprotective layer had ample smoothness, and it was unnecessary to conductpost-working such as polishing of the outer shape of the toner carrier.Further, the provision of an adhesive layer in an interface between thesolid surface layer portion and the magnetic field generating layercontributed to an improvement in the adhesive strength between themagnetic field generating layer and the solid surface layer portion. Theabovedescribed toner carrier having a magnetic field generating layerwas subjected to magnetization with a very fine pitch, and a thin layerof a toner comprising a magnetic toner could be held on the surface ofthe toner carrier by magnetic force. Further, when this toner carrierwas pressed against a latent image carrier having an outer diameter of60 mm with a pressure of 2 g/mm, the developing nip width (contactwidth) was about 2 mm. Thus, a developing device capable of maintaininga soft and stable pressure contact state was obtained. This developingdevice was used for an image forming apparatus as shown in FIG. 3, and aline image of 600 DPI, a character image and a solid image werecontinuously formed on 20,000 sheets of recording paper. As a result,the 600DPI line image having a high resolution was stably obtainedwithout a significant edge effect and without thickening of the lineimage, and a solid image having neither tailing nor fogging in the endof the image and having a good dot recording reproducibility, anexcellent area gradation and a high density of 1.4 or more in terms ofthe OD value was stably formed. Further, no fogging occurred on thelatent image carrier, not to mention on the recording paper, so that theamount of a waste toner was remarkably reduced. Thus, a reduction in thefogging by virtue of magnetic restraint was confirmed. Further, neitherunfavorable phenomena such as fixation and fusing on the latent imagecarrier or toner carrier nor damage to the toner was observed. Thus, thedurability of the toner carrier was improved by providing a protectivelayer.

Comparative Example 1

A shaft and a foam member of a silicone rubber foam were integrallymolded by reaction injection molding through the use of anelectrifying-agent-containing silicone rubber as a main startingmaterial to form an electroconductive toner carrier having an outerdiameter of 20 mm. It was necessary to conduct post-working such aspolishing of the outer shape of the toner carrier for the purpose ofremoving unnecessary portions, such as parting lines, to attain a highouter shape precision. The weight of the toner carrier except for theshaft could not be reduced and was 75 g when the toner carrier had alength of 230 mm and an outer diameter of 60 mm. When this toner carrierwas pressed against a latent image carrier having an outer diameter of60 mm by a pressure of 2 g/mm, substantially no developing nip width(contact width) could be obtained. In order to obtain a developing nipwidth (contact width) of about 1 mm, the toner carrier had to be pressedagainst the latent image carrier with a pressure of 20 g/mm or more.This made it necessary to increase the size of pressing mechanismcomponents, and the application of the pressure caused the latent imagecarrier, toner carrier and toner to be damaged. The above-describeddeveloping device was used for an image forming apparatus as shown inFIG. 1, and a line image of 600 DPI, a character image and a solid imagewere continuously formed on 10,000 sheets of recording paper. As aresult, the 600 DPI line image suffered from thickening of the lineimage, and only a solid image having severe fogging was obtained. Whenan image was formed on several tens of sheets, a flaw occurred on thelatent image carrier and the toner fixed on the latent image carrier orthe toner carrier, so that it was difficult to form a normal image.

The present invention is not limited to the abovedescribed examples onlybut can be widely applied to developing devices such aselectrophotography devices, particularly to printers, copying machines,facsimile machines and displays.

As described above, according to the present invention, a soft pressurecontact development can be stably conducted through the use of a softfoam member, which makes possible formation of an image having a highresolution and less susceptible to a fluctuation in the density.Further, it is possible to provide a developing device which is suitablefor pressure contact development and easy to manufacture, has a smallsize and a light weight and entails low cost.

In particular, when the toner carrier comprises a foam portion and asolid surface layer portion comprising an identical material continuouswith each other and the solid surface layer portion is used as a tonerholding surface, it becomes possible to prepare a toner carrier whichrequires no post-working and is inexpensive. Further, a soft pressurecontact development becomes possible through the utilization of thesoftness of the foam portion and the flexibility of the solid surfacelayer portion, so that it is possible to provide a developing devicewhich is less liable to give rise to damage to a toner and has highdurability.

The provision of an electroconductive layer on the solid surface layerportion makes a development electrode proximate to the latent imagecarrier, which enables an image having a high resolution to be formedwithout a significant edge effect.

The provision of a magnetic field generating layer on the solid surfacelayer portion enables the toner to be restrained on the toner carrier,so that the toner can be stably transferred and fogging can be preventedby virtue of magnetic force. This contributes to a reduction in theamount of waste toner.

When the toner carrier is in the form of a roller and a solid surfacelayer portion is formed at least on the outer periphery of the foammember, it is possible to obtain an outer shape face which requires nopolishing of the outer shape and makes it possible to easily formfunctional layers, such as an electroconductive layer and a magneticfield generating layer, on the solid surface layer portion.

The provision of a solid surface layer portion in a portion to be fixedto a shaft contributes to an improvement in the fixing strength betweenthe shaft and the foam member.

When the thickness of the solid surface layer portion is 2 mm or less,it is possible to obtain ample flexibility for pressure contactdevelopment.

When the foam member comprises a soft foam, ample pressure contact widthcan be obtained, by which the development density can be stabilizedthrough a reduction in the variation of the pressure contact state ofthe pressure contact portion.

What is claimed is:
 1. An image forming apparatus having a roller memberwhich is in pressure contact with a latent image carrier, said rollermember comprising:i) a shaft; ii) a flexible member bonded to saidshaft, said roller member comprising a foam forming material which isformed into at least two portions, said at least two portions includinga foam portion and a first solid portion, said foam and solid portionsbeing continuous with each other substantially without interface, saidsolid portion being spaced in a radial direction from said shaft withsaid foam portion lying between said solid foam portion and said shaft,said roller member having a density gradient in the radial directionsuch that said roller member exhibits in cross section a U-shapeddensity gradient curve.
 2. The image forming apparatus as claimed inclaim 1, wherein said apparatus comprises means for biasing said rollermember against the latent image carrier.
 3. The image forming apparatusas claimed in claim 1, wherein said roller member further comprises asecond solid portion which is bonded to said shaft.
 4. The image formingapparatus as claimed in claims 1, wherein said solid portion has athickness which is 2 mm or less.
 5. The image forming apparatus asclaimed in claim 4, wherein said foam portion has a thickness andwherein the thickness ratio of the solid portion to the foam portion isin the range of from about 0.01 to 0.5.
 6. A roller member, which is inpressure contact with a latent image carrier, said roller membercomprising:i) a shaft; ii) a flexible member bonded to said shaft, saidroller member comprising a foam forming material which is formed into atleast two portions, said at least two portions including a foam portionand a first solid portion, said foam and solid portions being continuouswith each other substantially without interface, said solid portionbeing spaced in a radial direction from said shaft with said foamportion lying between said solid foam portion and said shaft, saidroller member having a density gradient in the radial direction suchthat said roller member exhibits in cross section a U-shaped densitygradient curve; and iii) magnetic field generating means formed on thesolid portion of the roller member.
 7. The roller member as claimed inclaim 6, wherein said magnetic field generating means comprises amagnetic field generating layer on the solid portion, and wherein theroller member further comprises protective layer means on the magneticfield generating layer for enhancing the durability of the magneticfield generating layer.
 8. A roller member, which is in pressure contactwith a latent image carrier, said roller member comprising:i) a shaft;ii) a flexible member bonded to said shaft, said roller membercomprising a foam forming material which is formed into at least twoportions, said at least two portions including a foam portion and afirst solid portion, said foam and solid portions being continuous witheach other substantially without interface, said solid portion beingspaced in a radial direction from said shaft with said foam portionlying between said solid foam portion and said shaft, said roller memberhaving a density gradient in the radial direction such that said rollermember exhibits in cross section an S-shaped density gradient curve; andiii) electroconductive means formed on the solid portion of the rollermember.
 9. The roller member as claimed in claim 8, wherein saidelectroconductive means comprises an electroconductive layer on thesolid portion, and wherein the roller member further comprisesprotective layer means on the electroconductive layer for enhancing thedurability of the electroconductive layer.
 10. An image formingapparatus having a roller member, which is in pressure contact with alatent image carrier, said roller member comprising:i) a shaft; ii) aflexible member bonded to said shaft, said roller member comprising afoam forming material which is formed into at least two portions, saidat least two portions including a foam portion and a first solidportion, said foam and solid portions being continuous with each othersubstantially without interface, said solid portion being spaced in aradial direction from said shaft with said foam portion lying betweensaid solid foam portion and said shaft, said roller member having adensity gradient in the radial direction such that said roller memberexhibits in cross section an S-shaped or U-shaped density gradientcurve; and iii) magnetic field generating means formed on the solidportion of the roller member.
 11. The image forming apparatus as claimedin claim 10, wherein said magnetic field generating means comprises amagnetic field generating layer on the solid portion, and wherein theroller member further comprises protective layer means on the magneticlayer for enhancing the durability of the magnetic field generatinglayer.
 12. An image forming apparatus having a roller member, which isin pressure contact with a latent image carrier, said roller membercomprising:i) a shaft; ii) a flexible member bonded to said shaft, saidroller member comprising a foam forming material which is formed into atleast two portions, said at least two portions including a foam portionand a first solid portion, said foam and solid portions being continuouswith each other substantially without interface, said solid portionbeing spaced in a radial direction from said shaft with said foamportion lying between said solid foam portion and said shaft, saidroller member having a density gradient in the radial direction suchthat said roller member exhibits in cross section an S-shaped orU-shaped density gradient curve; and iii) electroconductive means formedon the solid portion of the roller member.
 13. The image formingapparatus as claimed in claim 12, wherein said electroconductive meanscomprises an electroconductive layer on the solid portion, and whereinthe roller member further comprises protective layer means on theelectroconductive layer for enhancing the durability of theelectroconductive layer.